Methods of phosphate chemical treatment have been used in the past in various fields including surface preparation treatment before point-coating, pretreatment prior to cold working, and the like.
For example, in Japanese Unexamined Patent Publication (Kokai) No. 60-208479 there is disclosed a method for acid phosphate chemical treatment of iron, steel, zinc and/or aluminum surfaces.
Also, Japanese Unexamined Patent Publication (Kokai) No. 64-68481 discloses a method for the phosphate chemical treatment of steel and/or galvanized steel, or of metals consisting of aluminum and steel and/or galvanized steel.
Also, Japanese Unexamined Patent Publication (Kokai) No. 2-190478 discloses a chemical treatment bath containing Fe.sup.+3 ion in a method of forming a phosphate film onto aluminum surfaces.
Also, in Japanese Unexamined Patent Publication (Kokai) No. 4-120294 there is disclosed a method of forming a phosphate chemical treatment coating as a surface preparation treatment before point-coating for stainless steel, in which the phosphate coating is formed by applying a PR (periodic reverse) pulse electric current to the stainless steel for electrolysis in a phosphate chemical treatment bath.
However, regarding methods for phosphate chemical treatment according to the prior art, there are many known methods of forming phosphate chemical coatings onto materials to be treated other than iron, as described in Japanese Unexamined Patent Publication (Kokai) No. 60-208479, Japanese Unexamined Patent Publication (Kokai) No. 64-68481 and Japanese Unexamined Patent Publication (Kokai) 2-190478, etc., but problems have been caused by the need to change the components of the phosphate chemical treatment bath and the conditions at the time of treatment, depending on the type of the material to be treated. In addition, the components and conditions for the phosphate chemical treatment bath are extremely critical, and not at all practical.
Furthermore, as in Japanese Unexamined Patent Publication (Kokai) No. 4-120294, the possibility has been known of forming phosphate chemical treatment coatings even onto materials to be treated other than steel, such as stainless steel, by subjecting the material to electrolysis in a phosphate chemical treatment bath, but such coatings are still limited to the formation of very thin films, such as surface preparation treatments before paint coating.
The present invention was accomplished with the object of overcoming the above mentioned problems, and its purpose is to provide a method for phosphate chemical treatment which makes it possible to produce a phosphate chemical coating of adequate film thickness onto any metal surfaces regardless of the degree of electric conductivity thereof.